The aim of this essay is to examine the relationship of ethnicity to nationalism by the works of two predominant authors who made considerable contributions in developing of theories related with the ethnicity and nationalism: Ernest Gellner and Anthony D. Smith. In this comparison based study, in order to examine the relationship of ethnicity to nationalism more effectively and efficiently, initially focused on different definitions and perceptions of the two authors about ethnicity, ethnic identity, nation and all these relationships with nationalism. In the further stages, essay focus on the comparison and the contrast of the Smith’s ethno-centric approach versus Gellner’s modernist approach. As a conclusion, this paper argued that although Gellner’s theory is quite convincible about the explanation of the; evaluation, social roots, force and power of nationalism in the modern world, Smith’s ethno-centric approach had better theoretical explanation on the relationship of ethnicity to nationalism.